Composite conduits, ducts or pipes are designed to convey corrosive exhaust fumes such as toxic, noxious and flammable vapors, dust or other contaminants, abrasive materials and the like. Such fumes or gases are typically generated within an industrial plant although they may also be used for odor control within a municipal facility exhaust system. In the usual case however, the various chemicals employed during the manufacturing process generate vapors that are removed from the plant via the plant air duct system.
In view of the above, the construction of the duct has to be such that the inner layer of the duct is rendered impervious to a wide range of acid/caustic/solvent fumes as well as to the condensation of toxic vapors on the inside surface of the duct or conduit. Hitherto, such conduits were manufactured out of metal and their alloys. As a result, conduits constructed from these alloys corrode over time. In more recent years, metal conduits lined with corrosion resistant polymers have been made available; however, limitations such as short flanged sections, retrofit difficulties, etc. make the installation costs prohibitive.
The increased emphasis in the recent years on air pollution control systems, has resulted in a need for replacement materials. Plastic ducts have gained tremendous ground in replacing the more expensive metal conduits. In particular, thermoplastic materials such as polyvinyl chloride and polypropylene as well as fiberglass reinforced plastic materials have been used extensively to fabricate fume exhaust ductwork. However, such materials are not fire resistant and therefore susceptible to burn and decomposition once exposed to heat and flames. Even if the exposure to flames is not continuous, many prior art plastic conduits, including those having a phenolic resin exteriors, emit toxic fumes, causing yet another hazard.
Frequently, the ductwork is located within highly inaccessible areas of the facility and therefore a sprinkler system will be required if the ductwork is not constructed from fire-resistant materials. Fires are caused for a variety of reasons; for example, spontaneous combustion, frictional heat, or static electricity. It is therefore increasingly necessary for the duct to be manufactured from materials having resistance to heat and fire.
Even though many plastics are said to be corrosion proof, continuous passage of corrosive fumes along the inner surface of the conduit will, over time, corrode and weaken the duct. In an effort to resist such chronic corrosion, plastic conduits are often reinforced with so-called "veils" that are impervious to various corrosive chemicals.
In the past few years, various types of thermosetting resins have been used such as isopthalic, orthopthalic, bisphenols, chlorendic anhydrides, epoxies and vinylesters. A problem common to all these materials is flammability, since they each burn rapidly and generate substantial amounts of smoke. Many attempts have been made to reduce the flammability of these materials. This is usually done through the addition of various chemicals and filler materials known to interfere with and thus retard combustion.
Up to the present time, no single conduit material has been able to resist all the various chemicals used in most industries nor provide a degree of fire retardance sufficient to obviate the need for a supplemental sprinkler system. As is apparent, considerable savings may be achieved with a duct system that does not require various internal sprinklers, drains and support members otherwise necessary to bring the duct up to required safety standards. A need has therefore existed in the art for a conduit system having high corrosion resistant properties while at the same time providing high resistance to fire and temperature yet does not emit toxic gases when exposed to such high temperatures.